8.3 Embryonic & Fetal Development, the Placenta, Fetal Circulation & Amniotic Fluid
Key Takeaways
Fertilization occurs in the ampulla, implantation begins about 6–7 days after fertilization, and the syncytiotrophoblast produces hCG detectable in maternal serum about 8–11 days after conception.
Organogenesis spans post-conception weeks 3–8 (about 5–10 weeks from the LMP), the period of greatest structural teratogen risk.
The placenta takes over progesterone production from the corpus luteum at about 7–10 weeks and actively transfers maternal IgG, most in the third trimester.
Fetal circulation uses three shunts (ductus venosus, foramen ovale, and ductus arteriosus); NSAIDs in late pregnancy can constrict the ductus arteriosus prematurely.
After about 16–20 weeks, amniotic fluid comes mainly from fetal urine and is removed by fetal swallowing, so renal agenesis causes oligohydramnios and esophageal atresia causes polyhydramnios.
From Fertilization to Implantation
- Fertilization occurs in the ampulla of the fallopian tube, usually within 12–24 hours of ovulation; sperm can survive up to about 5 days in fertile cervical mucus.
- The zygote divides into a morula (about day 3) and then a blastocyst (about day 5) with an inner cell mass (the embryo) and an outer trophoblast (the placenta).
- Implantation begins about day 6–7 after fertilization and is complete by about day 10. The trophoblast differentiates into the invasive syncytiotrophoblast, which secretes hCG, and the cytotrophoblast.
- hCG maintains the corpus luteum and is detectable in maternal serum about 8–11 days after fertilization, roughly when the next period is due.
Developmental Periods and Milestones
Embryologists count from conception; clinicians count gestational age from the LMP, about 2 weeks earlier.
| Period | Post-Conception Timing | Gestational Age (from LMP) | Key Events |
|---|---|---|---|
| Pre-embryonic ("all-or-none") | Weeks 1–2 | Weeks 3–4 | Teratogens either cause loss or have no effect |
| Embryonic (organogenesis) | Weeks 3–8 | Weeks 5–10 | Neural tube closes by day 28 after conception; heart beats by about 6 weeks GA; limbs, face, and palate form; greatest structural teratogen risk |
| Fetal | Week 9 to birth | Week 11 to term | Growth and functional maturation; teratogens cause growth restriction or functional deficits |
Other milestones (gestational age): Fetal thyroid begins functioning at about 10–12 weeks (before then the embryo depends on maternal thyroxine). External genitalia become distinguishable on ultrasound by about 12–14 weeks. Quickening is felt at 16–20 weeks. Surfactant production begins at about 24 weeks and is usually adequate by 34–36 weeks. Periviability is about 22–25 weeks.
The Placenta
At term the placenta weighs about 500 g. Maternal blood enters the intervillous space from spiral arteries, which trophoblast remodels into low-resistance vessels; failure of this remodeling underlies preeclampsia and fetal growth restriction.
Transfer Mechanisms
| Substance | Mechanism |
|---|---|
| Oxygen, CO2, and small lipophilic drugs | Simple diffusion |
| Glucose | Facilitated diffusion (GLUT1), so maternal hyperglycemia passes to the fetus |
| Amino acids, calcium, iron | Active transport |
| IgG antibodies | Receptor-mediated transport (FcRn), mostly in the third trimester; this is how Tdap, influenza, COVID-19, and RSV vaccines protect newborns, and how maternal anti-D, anti-Ro, and thyroid antibodies reach the fetus |
| Insulin, heparin, and large molecules | Generally do not cross; this is why insulin and low-molecular-weight heparin are preferred in pregnancy |
Placental Hormones
- hCG: Peaks at 8–10 weeks; sustains the corpus luteum and stimulates the TSH receptor (which can cause transient gestational hyperthyroxinemia).
- Human placental lactogen: Rises through pregnancy and creates the insulin resistance behind gestational diabetes.
- Progesterone: From the corpus luteum until the luteal-placental shift at about 7–10 weeks, then from the placenta; maintains uterine quiescence.
- Estriol: Made by the placenta from fetal adrenal DHEA-S converted in the fetal liver, so it reflects fetal and placental health (a quad-screen marker).
The Umbilical Cord
The cord normally contains two arteries (carrying deoxygenated blood from the fetus to the placenta) and one vein (carrying oxygenated blood to the fetus), cushioned by Wharton's jelly. A single umbilical artery (about 0.5%–1% of births) is linked to fetal anomalies and growth restriction, so it warrants a detailed anatomy scan and growth surveillance. In a velamentous insertion, vessels run unprotected through the membranes, raising the risk of vasa previa.
Fetal Circulation
- Oxygenated blood travels from the placenta through the umbilical vein.
- About half bypasses the liver through the ductus venosus into the inferior vena cava.
- In the right atrium, most of this oxygen-rich blood crosses the foramen ovale to the left atrium and left ventricle, supplying the brain and coronary arteries.
- Blood from the superior vena cava goes to the right ventricle and pulmonary artery, where most bypasses the high-resistance lungs through the ductus arteriosus into the descending aorta.
- Fetal hemoglobin has a higher oxygen affinity than adult hemoglobin, which helps the fetus extract oxygen.
At birth, lung expansion drops pulmonary vascular resistance, left atrial pressure rises and closes the foramen ovale, and rising oxygen and falling prostaglandins close the ductus arteriosus within days. Clinically, NSAIDs such as indomethacin can constrict the ductus arteriosus prematurely when used after about 30–32 weeks, which is why their use in pregnancy is time-limited.
Amniotic Fluid
- Early fluid is a transudate of maternal plasma; after about 16–20 weeks, fetal urine is the main source, and fetal swallowing and lung secretions handle turnover.
- Volume peaks at about 800–1,000 mL around 34–36 weeks, then declines.
- Oligohydramnios (deepest vertical pocket <2 cm or AFI ≤5 cm): renal agenesis or obstruction, PPROM, placental insufficiency, or post-term pregnancy; prolonged early oligohydramnios causes pulmonary hypoplasia and limb contractures.
- Polyhydramnios (deepest pocket ≥8 cm or AFI ≥24 cm): maternal diabetes, impaired fetal swallowing (esophageal or duodenal atresia, anencephaly), fetal anemia, or idiopathic causes.
Twinning and Chorionicity
Dizygotic twins (two eggs) are always dichorionic-diamniotic. In monozygotic twins, chorionicity depends on when the embryo divides:
| Timing of Division After Fertilization | Placentation |
|---|---|
| Days 0–3 | Dichorionic-diamniotic |
| Days 4–8 | Monochorionic-diamniotic (risk of twin-twin transfusion syndrome) |
| Days 8–12 | Monochorionic-monoamniotic (cord entanglement) |
| After about day 13 | Conjoined twins |
First-trimester ultrasound is the best time to determine chorionicity (lambda sign for dichorionic; T sign for monochorionic), and chorionicity sets the surveillance plan.
A pregnant patient at 32 weeks asks why her clinician recommended the Tdap vaccine now rather than earlier in pregnancy. Which physiologic principle best explains the timing?
Maternal IgG crosses the placenta by active transport, mostly in the third trimester, so 27–36 weeks maximizes transfer.
The fetus begins producing its own IgG at 28 weeks and needs vaccine antigen to cross the placenta at that time.
IgM antibodies made after vaccination cross the placenta easily only after 30 weeks, so earlier doses are wasted.
The vaccine itself crosses the placenta and immunizes the fetus directly once its thymus matures at 32 weeks.
At an anatomy ultrasound at 20 weeks, the cord is noted to have only two vessels: one artery and one vein. What is the most appropriate next step?
Recommend immediate delivery because the fetus is at imminent risk of demise.
Reassure the patient that a two-vessel cord is a normal variant needing no follow-up.
Perform a detailed anatomic survey and plan serial growth surveillance.
Prescribe low-dose aspirin to restore umbilical blood flow.
A 26-year-old at 34 weeks takes over-the-counter ibuprofen several times daily for back pain. Which fetal effect is the greatest concern with continued NSAID use at this gestational age?
Neural tube defects from impaired folate metabolism
Premature constriction of the ductus arteriosus
Fetal macrosomia from increased placental glucose transfer
Transposition of the great arteries in the fetal heart
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